1. Field of the Invention
The present invention relates to an intake port structure for a cylinder head of an engine and a method of determining the intake port structure.
2. Description of Related Art
Much attention has been given to various fluid mechanical approaches relating to techniques for determining configurations of intake ports and, in particular, the configurations of portions of the ports in close proximity to valve seats. Such configurations are one of the most important and decisive factors governing the flow pattern of intake air in cylinders. Typically, such an intake port configuration is designed and specified with regard to the center line and upper and lower profiles of an open end of the intake port only and finished, in actual fact, through trial and error. Various efforts have been made to configure the intake port in order to produce a swirl in a plane perpendicular to the axis of the cylinder and a vertical "tumble" in a plane parallel to the axis in a combustion chamber. One such effort is that described in Japanese Unexamined Utility Model Publication No. 4-137224. In order for the intake port to produce an enhanced tumble, the intake port is made, according to this publication, so as to be directed to and meet the outlet opening into the combustion chamber at a small angle (hereafter referred to as a directional angle). The intake port is further made to have an approximately straight center line intersecting the axis of an intake valve and a cross-section which is enlarged toward a location at which a tumble is produced. Another effort, described in Japanese Unexamined Patent Publication No. 5-340258, involved providing two intake ports, namely a primary intake port, which meets an outlet end opening into the combustion chamber at a small directional angle, and a secondary intake port, which meets the combustion chamber at a large directional angle and is selectively used according to engine operating conditions.
An intake port which meets the combustion chamber at a small directional angle, relative to a plane along which an outlet end opens into the combustion chamber, is advantageous in that it enhances a swirl in the combustion chamber. Nevertheless, intake air flow forced toward a tangential direction of the cylinder is deflected toward the center axis of the cylinder due to a curved transitional portion between the major portion of the intake port and the outlet end or valve port. This deflection leads to aggravation of a swirl. If the intake port does not have such a curved transitional portion and is straight and intersects the center axis of the cylinder, it is, practically, difficult to provide for an adjoining portion, between the straight portion of the intake port and the valve port, a cross-sectional configuration having a resistance which is sufficiently low for intake air to flow smoothly. Further, it is typical, in designing an intake port, to define a center line and inlet and outlet ends of the intake port only. Because details of the intake port configurations are not specified, it is impossible to simulate and analyze flow patterns of intake air. Consequently, the hydromechanical study of such an intake port takes a lot of time and it is difficult to improve the configuration of the proposed intake port in a short period.